Sensory processing in the central nervous system involves both ascending and descending pathways. For example, ascending nociceptive impulses which mediate oral-facial pain, are influenced by descending control systems originating mainly in the brain stem. In the visual system, ascending impulses from the retina are influenced by descending control systems originating in the cortex. The proposed research examines both a descending modulatory pathway and an ascending sensory pathway. Phase I. Fibers descending to the superior colliculus arise from a wide expanse of prestriate, temporal and parietal cortex. Electrophysiological and behavioral studies in monkeys indicate that this corticotectal system plays an important modulatory role in visuomotor integration. Individual components of this system descend from each of the known subdivisions of visual cortex. Corticotectal cells of each component may form a unique subpopulation of cells within the visuotopic area from which they originate. Our goal in Phase I is to examine one such component namely, the corticotectal pathway from extrastriate area MT. The visual response properties of individual corticotectal cells in MT will be examined with microelectrode recordings in the anesthetized monkey, and compared to a control population of MT cells which do not project to the colliculus. Phase II. Ascending nociceptive information from the oral cavity is relayed to the thalamus by long projection neurons in the superficial layers and lamina V of the medullary dorsal horn. Our goal in Phase II is to determine whether unmyelinated primary afferents synapse on trigeminothalamic cells of lamina V. Trigeminothalamic neurons, identified by antidromic activation from the contralateral thalamus, will be filled with horseradish peroxidase (HRP), and terminals of unmyelinated primary afferent fibers will be labeled with orthogradely transported HRP injected into the dorsal root ganglion. The morphological features of filled cells will be described and the synaptic interaction between these cells and labeled terminals will be analyzed.